Troubleshooting Guide
PVC Extrusion of rigid PVC pipes & profiles
Brittle extrudate
Cause
Solution
Poor fusion
Increase melt temperature, check the level of filler.
Insufficient impact modification
Increase the amount of impact modifiers.
Voids and contamination
See "section C & L".
Cold die lines
Increase melt temperature.
Over processing
Identify optimum process temperature.
Excessive shrinkage
Cause
Solution
Poor match between die and calibrator
Increase diameter of pin.
Reduce line speed.
Foreign objects in blend
Cause
Solution
Contamination in feed stock
Use hopper magnet for metallic contamination.
Consider sieving.
Poor acetone or methylene chloride test
Cause
Solution
Material too cold and not fully fused
Barrel and screw temperatures too low. Increase melt
temperature.
Formulation may be over-lubricated
Decrease external lubricant.
Add or increase process aid to promote fusion.
Wavy or lumpy internal surface
Cause
Solution
Wavy or lumpy internal surface is indicative of
different thermal or shearing histories in the melt
The screw and barrel in the metering zone should be
about the same temperature. A little higher heat in
both will help smooth out the internal surface.
Low gloss
Cause
Solution
Over lubrication
Reduce internal lubrication.
Melt too cold
Increase process temperature.
Poor gelation
Increase process aid level.
Not fully sized
Increase vacuum or increase air pressure on plug to
insure fully contact with sizing sleeve.
Possible inaccuracy in the mix blend
Check the mix room scales for accuracy.
Rough (orange peel) internal surface or outer surface leading to burn
Cause
Solution
Over heating
Reduce spider and cone heat zones.
Lower melt temperature.
Formulations may be under lubricated
Check formulation. Increase lubrication.
Uniform yellowing or discoloration of material
Cause
Solution
Level of stabilizer too low
Check level and adjust formulation if required.
Melt temperature excessive
Lower overall melt temperature.
Incorrect lubrication
Increase lubrication.
Rear barrel too hot
Reduce rear barrel temperature.
Visible spider lines on internal diameter
Cause
Solution
Spider and die heating zone may be too cold
Check operating temperatures.
Material may be over lubricated
Adjust lubrication.
Non-uniform outer diameter
Cause
Solution
Sizing sleeve may have build-up / plate out of
pigments, lubricants etc
Mineral spirits added drop-wise on hot melt just as it
enters vacuum sizing sleeve usually will remove most
of the plateout. Otherwise line should be stopped and
cleaned up.
On large pipe, die and head may not be uniformly
heated at the start of the run or heaters may be
hunting for set temperatures, controllers
It will take several hours to heat soak the large mass
of metal. Allow adequate time to heat up properly.
Also check heater bands and thermocouple and PID
controllers.
Pipe not round
Cause
Solution
Insufficient sizing
Check for full sizing for pressure or sufficient vacuum.
Cooling water not cold enough
Use a chiller for increase cooling.
The haul off may be applying excessive pressure
Lower the haul off pressure.
Inefficient cooling
On large pipes cascade or spray cooling is more
efficient than flooded tank.
Worn out plug
If this problem continues after a long run, replace plug
to insure no loss of air pressure for sizing.
Bubbles/voids
Cause
Solution
Degradation
Check barrel, screw, and die temperature control. Use
screw cooling. Reduce melt temperature.
Moisture or trapped air
Check material is not gelled at vent, check vacuum
pump if not blocked.
Internal diameter exhibits occasional tearing
Cause
Solution
Plug too close to die pulling material apart before it is
cooled sufficiently
Use longer chain – at last 75% of the cooling tank
length.
Bridging in the throat section / non-uniform powder flow to the screw
Cause
Solution
Poor flow at the hopper throat
Turn the cooling water on at the throat section.
Use a crammer in the hopper.
Install a vibrator to encourage powder flow.
High amps / overloading
Cause
Solution
Material too cold
Increase barrel temperature.
Check barrel thermocouples for accuracy.
Under lubrication
Increase external lubricant.
Over feeding
Increase main screw speed or reduce feeder speed.
Check die is appropriate for the extruder.
Thin wall pipe in vacuum sizing collapsing, or jamming up, or drawing down too thin
Cause
Solution
Extrudate unstable entering into vacuum tank
Decrease the distance. Two to three inches between
the die and the tank should be sufficient.
Melt too hot
Reduce die temperature.
Pulling friction too high.
Lower the vacuum.
Non-uniform wall thickness around the pipe circumference
Cause
Solution
Thick thin wall opposite each other
Bushing is not centered. Adjust die bolts for even flow
out of die.
Thick thin opposite each other
Use segmented die heaters to adjust flow.
Increasing or decreasing wall thickness
Cause
Solution
Feed stock inconsistencies between batches or mixes
Adjust puller speed. Monitor feedstock consistency.
Bridging at the hopper
See section "high amps / overloading".
Surging
Cause
Solution
Partial plugging of screw channel with solids du to
excessive fillers i.e. not enough polymer to allow
fluxing
Raise process temperatures, adjust formulation.
Inconsistent feed rate
Check the feeding system.